1. Field of the Invention
The present invention relates to an electron beam apparatus, such as a scanning electron microscope enabling imaging of a specimen under low-vacuum condition.
2. Description of Related Art
FIG. 1 is a schematic cross section of main portions of a scanning electron microscope capable of low-vacuum imaging of specimens. In FIG. 1, the microscope has an electron optical column 1 where there is formed a beam passage 4 through which an electron beam 5 travels. The passage 4 is a tubular passageway. The beam 5 transmitted through the passage 4 is released from the front end of the electron optical column 1 via the front end of an objective lens 3 disposed in a front-end portion of the electron optical column 1. An electron gun acting as an electron beam source, deflection coils, and other components (none of which are shown) are mounted in the column 1.
The front-end portion of the electron optical column 1 is attached to a specimen chamber 2. The beam 5 released from the front end of the column 1 is made to impinge on a specimen 6 placed within the specimen chamber 2. At this time, the beam 5 impinging on the specimen 6 is deflected by the deflection coils and scanned over the specimen 6.
During scanning of the electron beam 5, a scan current based on a given scan signal is supplied to the deflection coils to activate the coils, thus deflecting the beam 5.
Electrons (not shown) to be detected, such as backscattered electrons, are produced from the specimen 6 irradiated with the electron beam 5. The electrons evolved from the specimen 6 in this way are detected by an electron detector (not shown) mounted in the specimen chamber 2. A scanned image of the specimen 6 is created based on a detection signal produced from the detector and on the scan signal, and the image is displayed. An operator who manipulates the scanning electron microscope can observe the specimen by visually checking the displayed image. When low-vacuum imaging of the specimen 6 is performed in this way, backscattered electrons more energetic than secondary electrons are often detected.
When low-vacuum imaging of the specimen 6 is performed, it is necessary that the vacuum ambient inside the specimen chamber 2 where the specimen 6 is placed be made lower in degree of vacuum than the vacuum ambient inside the beam passage 4 in the electron optical column 1; that is, made a high-vacuum ambient. (The pressure of a “high-vacuum” condition is lower than the pressure of a “low-vacuum” condition.) Accordingly, the scanning electron microscope for low-vacuum imaging as shown in FIG. 1 is so designed that differential pumping is effected between the inside of the beam passage 4 and the inside of the specimen chamber 2 to maintain the low-vacuum ambient inside the specimen chamber 2 while retaining the high-vacuum ambient inside the beam passage 4 of the electron optical column 1.
In particular, an aperture 8 is disposed in the front-end portion of the beam passage 4 of the electron optical column 1, the aperture 8 being provided with an opening 8a for the differential pumping. The inside of the beam passage 4 and the inside of the specimen chamber 2 are pumped down by their respective vacuum pumping systems. At this time, the inside of the beam passage 4 is set to a high degree of vacuum. On the other hand, the inside of the specimen chamber 2 is set to a lower degree of vacuum.
The opening 8a in the aperture 8 is made so small that the low-vacuum ambient inside the specimen chamber 2 hardly affects the high-vacuum ambient inside the beam passage 4. However, the beam 5 can pass through the opening 8a. 
In the configuration shown in FIG. 1, the aperture 8 is placed on an aperture holder 7 that is composed of a convex portion and a body portion 7b. The aperture 8 is held on the top surface of the convex portion, which is located within the beam passage 4 in a front-end portion of the objective lens 3 of the electron optical column 1. Consequently, the aperture 8 held on the top surface of the convex portion is placed within the beam passage 4 in the front-end portion of the objective lens 3.
A through-hole 7a is formed in the aperture holder 7 and extends through the convex portion and body portion 7b of the aperture holder 7. Consequently, the beam 5 traveling in the beam passage 4 of the electron optical column 1 passes through the opening 8a in the aperture 8 and through the through-hole 7a in the holder 7 and reaches the specimen 6 in the specimen chamber 2.
In the specimen chamber 2, the aperture holder 7 is placed at the front end of the objective lens 3 as shown and supported by plural springs 9. That is, one end of each spring 9 is mounted to the body portion 7b of the aperture holder 7, while the other end is anchored to a support block 10 held to the top inner wall surface of the specimen chamber 2. In consequence, the aperture holder 7 is supported within the specimen chamber 2 via the springs 9 anchored to the support blocks 10.
Some known apparatus have lifting devices for moving up and down the differential pump and a horizontal drive mechanism for moving the differential pumping aperture horizontally after the aperture has descended, in addition to the support structure (FIG. 1) holding the aperture 8 (see JP-A-2008-010177).
In the scanning electron microscope adapted for low-vacuum imaging and designed as shown in FIG. 1, the aperture holder 7 holding the aperture 8 is held in the specimen chamber 2 via the springs 9 as described previously. In this support structure, the aperture holder 7 is kept held inside the specimen chamber 2. If the imaging mode is switched to high-vacuum imaging of the specimen 6, the aperture 8 stays intact in the beam passage 4.
Under this condition, even during high-vacuum imaging mode after switching of the imaging mode, the field of view and the imaging position are limited by the existence of the aperture 8 in the same way as in low-vacuum imaging mode. That is, the area of the specimen irradiated with the electron beam 5 directed from the front end of the electron optical column 1 (i.e., the front end of the objective lens 3) toward the specimen 6 is affected by the size of the opening 8a in the aperture 8. As a result, the area illuminated by the beam 5 is restricted. Consequently, the field of view and imaging position on the specimen 6 are restricted during imaging.
Accordingly, when the imaging mode is switched from low-vacuum imaging to high-vacuum imaging, it is necessary to retract the aperture 8 from the front end of the electron optical column 1 such that the opening 8a in the aperture 8 completely gets out of the path of the beam 5 by moving the aperture holder 7. However, in the structure of the apparatus described above, the following sequence has been necessary. That is, the internal ambient of the specimen chamber 2 is once restored to atmospheric pressure. Then, one sidewall 2a of the specimen chamber 2 is opened to open the inside of the specimen chamber 2. The springs 9 are removed from the support block 10 through the opening made in this way by a manual work of an operator. The holder 7 and aperture 8 are manually taken out of the specimen chamber 2 together with the springs 9. Thereafter, the sidewall 2a is moved to close the opening. Then, the inside of the specimen chamber 2 is pumped down to a desired high degree of vacuum.
When the aperture 8 is located within the beam passage 4, if the electron beam 5 hits a region around the opening 8a in the aperture 8, the aperture 8 is contaminated. In this case, the aperture 8 needs to be replaced as appropriate. However, in this replacing operation, too, a long sequence is needed. That is, after the springs 9 are removed from the support block 10, the aperture holder 7 and aperture 8 are removed from the front end of the objective lens 3. The holder 7 and aperture 8 are manually taken out of the specimen chamber 2 together with the springs 9. Then, the aperture 8 is replaced.
In the apparatus described in JP-A-2008-010177, the differential pumping aperture can be attached and detached from the objective lens without restoring the pressure inside the specimen chamber to atmospheric pressure. However, in this apparatus, when the aperture is removed from the objective lens, the operator manually manipulates the knob mounted on an external surface of the specimen chamber to lower the aperture in the vertical direction and then to move it horizontally. In this way, some manual operations are performed. During this process, it is necessary that the operator hold out his or her hand to the specimen chamber and perform some manual operations. There remains room for improvement for enhancing the controllability.
Furthermore, in the above-described structure that needs two stages of operations (i.e., a vertical motion and then a horizontal motion), if a discussion is effected to automate mounting and dismounting of the aperture using a drive means, such as a motor, it is conceivable that the structure of the apparatus will be complicated.